In recent years, because of consideration for global environment and the like, the social requirement of weight reduction of a vehicle such as an automobile and the like has been increasing more and more. In order to respond to such requirement, as a material for a large body panel (outer panel, inner panel) of an automobile panel such as a hood, door, roof and the like in particular, instead of a steel material such as a steel sheet and the like, application of an aluminum alloy material excellent in formability and baking finish hardenability and lighter in weight has been increasing.
Among them, for a panel such as an outer panel (outer sheet), inner panel (inner sheet) and the like of a panel structural body such as a hood, fender, door, roof, trunk lid and the like of an automobile, as a thin and high strength aluminum alloy sheet, use of an Al—Mg—Si-based AA or JIS 6000 series (may also be hereinafter simply referred to as a 6000 series) aluminum alloy sheet has been studied.
This 6000 series aluminum alloy sheet contains Si and Mg as inevitable elements, and an excess-Si type 6000 series aluminum alloy in particular has a composition of 1 or more Si/Mg mass ratio and has excellent age-hardenability. Therefore, it has baking finish hardenability (may also be hereinafter referred to as bake hardenability=BH properties and bake hardenability) with which formability is secured by lowering the proof stress in press forming and bending work, age hardening occurs by heating in artificial aging (hardening) treatment at a comparatively low temperature such as bake finish treatment of a panel after forming and the like to improve the proof stress, and the strength required as a panel can be secured.
Also, in the 6000 series aluminum alloy sheet, the alloy element amount is comparatively less in comparison to other 5000 series aluminum alloy and the like with much alloy amount of Mg amount and the like. Therefore, when the scrap of these 6000 series aluminum alloy sheets are reused as an aluminum alloy melting material (melting raw material), an original 6000 series aluminum alloy ingot is easily obtained, and recycling performance is also excellent.
On the other hand, as is known well, an outer panel of an automobile is manufactured by subjecting an aluminum alloy sheet to combined forming work such as stretch forming, bending forming and the like in press forming. For example, in a large outer panel such as a hood, door and the like, the shape is made a formed product shape as an outer panel by press forming such as stretching and the like, then joining with an inner panel is executed by hem working (hemming) of a flat hem and the like of the outer panel peripheral section to be formed into a panel structural body.
Here, the 6000 series aluminum alloy had an advantage of having excellent BH properties, but had a problem of having aging property at room temperature, age hardening in retention at room temperature for several months after solution heat treatment and quenching treatment to increase the strength, and thereby deteriorating the formability into a panel particularly the bending workability. For example, when a 6000 series aluminum alloy sheet is to be used for an automobile panel use, the sheet is placed at room temperature (left at room temperature) for approximately 1-4 months normally after solution heat treatment and quenching treatment (after manufacturing) at an aluminum manufacturer until forming work into a panel at an automobile manufacturer, and comes to be significantly age hardened (room temperature aging) during that time. Particularly in the outer panel subjected to severe bending work, there was such a problem that, although forming was possible without any problem after the lapse of 1 month after manufacturing, a crack occurred in hem working after the lapse of 3 months. Therefore, in the 6000 series aluminum alloy sheet for an automobile panel particularly for an outer panel, it is necessary to suppress room temperature aging over a comparatively long period of approximately 1-4 months.
Also, when such room temperature aging is large, such a problem also occurs that the BH properties deteriorate and the proof stress does not improve to the strength required as a panel by heating at the time of artificial aging (hardening) treatment at a comparatively low temperature such as baking finish treatment and the like of the panel after forming described above.
Therefore, with respect to improvement of the BH properties and suppression of room temperature aging of the 6000 series aluminum alloy, various proposals have been made from the past. For example, in Patent Literature 1, a proposal has been made in which strength variation at room temperature from the lapse of 7 days to 90 days after manufacturing is suppressed by changing the cooling rate stepwise at the time of solution heat treatment and quenching treatment. Also, in Patent Literature 2, a proposal has been made in which the BH properties and shape fixability are secured by being retained for 10-300 min at 50-150° C. temperature within 60 min after solution heat treatment and quenching treatment. Further, in Patent Literature 3, a proposal has been made in which the BH properties and shape fixability are secured by stipulating the cooling rate of the first step and the cooling rate thereafter at the time of solution heat treatment and quenching treatment.
Also, in Patent Literature 4, improvement of the BH properties in heat treatment after solution heat treatment and quenching treatment has been proposed. In Patent Literature 5, improvement of the BH properties by the stipulation of the endothermic peak of a DSC (Differential scanning calorimetry) method has been proposed. In Patent Literature 6, improvement of the BH properties by the stipulation of the exothermic peak of the same DSC has been proposed.
However, in these Patent Literatures 1-6, with respect to the cluster (aggregate of atoms) directly affecting the BH properties and aging property at room temperature of the 6000 series aluminum alloy sheet, the behavior thereof was merely indirectly reasoned by analogy.
On the other hand, in Patent Literature 7, a trial has been made in which the cluster (aggregates of atoms) affecting the BH properties and aging property at room temperature of the 6000 series aluminum alloy sheet has been directly measured and stipulated. More specifically, the average number density of the clusters whose circle equivalent diameter is within the range of 1-5 nm out of the clusters (aggregates of atoms) observed in analysis of the microstructure of a 6000 series aluminum alloy sheet by a transmission electron microscope of 1,000,000 magnifications has been stipulated in the range of 4,000-30,000 pieces/pmt to obtain one with excellent BH properties and suppressed room temperature aging.